1. Field of the Invention
The present invention relates to a method for fabricating titanium indiffusion (Ti-indiffusion) waveguides in optical modulators using a deuterium oxide (D2O) diffusion process and the resulting waveguides and modulators. More particularly, the present invention relates to a method for fabricating lithium niobate or lithium tantalate modulators by bubbling oxygen (O2) through deuterium oxide to create a wet environment for the Ti-indiffusion process.
2. Background
Optoelectronic components can be fabricated on several types of substrates including polymers, glass, semiconductors (e.g., gallium arsenide (GaAs) and indium phosphide (InP)) and inorganic materials (e.g., lithium niobate (LiNbO3) and lithium tantalate (LiTaO3)). Characteristically, an electro-optic material is one in which the index of refraction changes with the application of an electric field. One of the most important components in optoelectronic systems is the modulator. Three competing technologies in this realm are: direct modulation of a semiconductor laser, semiconductor electro-absorption modulators, and the lithium niobate modulator. Currently, lithium niobate modulators are the modulation devices of choice for many systems because they yield high performance, are a relatively mature technology and other modulation schemes impose limits not faced with lithium niobate modulators.
Lithium niobate has proven to be a suitable medium for components such as amplitude modulators, phase modulators, optical switches, polarization transformers, tunable filters and wavelength-selective optical add/drop filters. Lithium niobate has also been used as the host for solid state lasers using rare earth ions, e.g., erbium. Most current telecommunication and cable television system applications for LiNbO3 modulators involve discrete components for the optical transmitter subsystem. This configuration couples continuous wave lasers, typically diode-pumped YAG or erbium fiber oscillators, with lithium niobate external modulators and various wavelength and power stabilization components.
Lithium niobate is a popular nonlinear optical crystal for several reasons including its large electro-optic coefficients, the ease with which high quality optical waveguides are fabricated and its amenability to conventional integrated circuit processing techniques. High quality optical waveguides are those that possess low loss and relatively high power handling capabilities. Additionally, LiNbO3 is a hard material, thus it is easily polished for optical fiber coupling which makes its use in optical network systems relatively uncomplicated. It is also a relatively inexpensive crystal, due in part to its long history of use in surface-acoustic-wave (SAW) filters for radio frequencies. By comparison, lithium tantalate LiTaO3 is essentially interchangeable with lithium niobate as far as modulator characteristics are concerned, but the use of LiTaO3 is often cost prohibitive because it is not as widely commercially used as LiNbO3. Additionally, other optical crystalline structures having the formula RMO3, where R is an alkaline earth metal, M is a Group IVB or Group VB metal, and O is oxygen can conceivably be used in the fabrication of waveguides and modulators.
For example, one type of modulator incorporating the use of LiNbO3 is a Mach-Zehnder modulator. In a Mach-Zehnder modulator an incoming optical beam is split equally at a Y junction into two parallel waveguides, which then recombine at another Y junction after some distance. Electrodes are used to apply an electric field in the region of the optical waveguides. The electric field causes a relative phase shift between the two branches. If the phase shift in both arms is the same, then the two optical signals will recombine constructively at the second Y branch and no optical power will be lost. If there is a phase difference between the two arms, then there will be destructive interference and some optical power will be lost. The resulting destructive and constructive interference causes the output optical intensity to vary between a minimum and a maximum value.
In other electro-optic applications optical waveguide directional couplers can also be used as electro-optic modulators. In this type of modulator two waveguides are placed on the lithium niobate substrate in very close proximity to one another. The wave from one guide can “leak” into the other guide. By applying an electric field to the lithium niobate, the amount of “leakage” can be controlled, thus modulating the transfer of power from one guide to the other. Currently, different commercial application requirements favor either directional couplers or Mach-Zehnder modulators.
The advantage of high-speed, large bandwidth, digital or analog communications has led to a demand for the external modulator configuration. The most common approach utilizes a low-noise, high power diode-pumped laser whose signal is sent to the LiNbO3 modulator via optical fiber. The modulator applies either analog or digital information to the optical carrier.
When using lithium niobate in the fabrication of optical waveguides and optical modulators it is desired to avoid having a niobium-rich, lithium-poor and/or oxygen poor composition. When bulk lithium niobate has such niobium rich-compositions, and is then processed at high temperatures (T>300 degrees Celsius), growth of the LiNb3O8 phase in the crystal may occur. This phase is undesirable because it is not optically transparent and leads to high losses in optical waveguides and optical modulators.
Such niobium-rich compositions can occur in two different manners during fabrication of optical waveguides and optical modulators. First, typical ion exchange procedures result in the replacement of lithium atoms in the crystal lattice with a diffusion ion leading to a lithium niobate composition relatively rich in niobium. Second, standard high temperature (temperatures in excess of 300 degrees Celsius) processing of lithium niobate can cause Li2O out-diffusion, and result in niobium-rich, and lithium and oxygen poor compositions.
Ti-indiffusion waveguides are one of the more commonly used optical waveguides currently in existence. The Ti-indiffusion process drives titanium atoms into the crystal structure lattice causing titanium atoms to either disperse interstitially in the lattice or replace niobium atoms. This process results in one or more refractive indices (e.g., the extraordinary and ordinary refractive indices) of the crystal substrate being altered in the region where the titanium indiffusion occurs to create an optical waveguide.
Ti-indiffusion implements high temperature processing (temperatures in the range of 900 to 1100 degrees Celsius). To eliminate the undesired LiNb3O8 phase from forming in the crystal during Ti-indiffusion processing it is usually performed in a wet atmosphere of inert carrier gas, such as nitrogen (N2) or argon (Ar2), or in a wet atmosphere of oxygen (O2). This type of indiffusion process involves bubbling the inert carrier gas or oxygen gas through water (H2O). The wet atmosphere has been considered beneficial in the past because the H2O breaks down into H+ and OH− ions which chemically attack the LiNb3O8 phase, and break it back down into LiNbO3. The present inventors have discovered that a drawback of this type of high temperature processing is that the H2O gives off undesirable protons (H+) which are attracted by the lithium niobate and result in an inadvertent proton-exchanged surface layer occurring. These protons remain in the modulators after fabrication and flow relatively freely among the waveguides, the electrodes, the lithium niobate crystal and the buffer layer interface. It is now understood that these free flowing protons can and do adversely affect a modulator's DC-bias stability because they act as charge carriers and are driven by the applied electric fields, causing the response of the final product to drift over time when a bias is applied.
It would therefore be highly advantageous to devise a fabrication method for Ti-indiffusion waveguides that uses an improved flowing, wet atmosphere Ti-indiffusion process that does not introduce significant numbers of free flowing protons into the waveguide construct that adversely affect the waveguide/modulator's DC-bias stability.